Two antenna should be a ½ wave apart but on larger trucks if they are at least 9 feet apart (¼ Wave length) they will work OK.
Any closer and the overlap between the two antenna starts to reduce the actual transmission distance.
Also, when tuning the antennas, you need to tune each one separately for the lowest SWR using a piece of 50 ohm coax approximately the same length as one leg of the Co-Phased Harness, then use a good quality Coaxial Co-Phased Harness to bring it all together. (the harness is made with 75 ohm coax)
As stated the Tool Box is a poor mounting location unless it has a good ground and is coupled to the body/frame electrically then the antenna should work OK. How far the mobile setup will actually transmit is based on a lot more than the ground plane of the vehicle.
The best location, as we have heard before, is as high as possible and centered on the vehicle.
The best mounting type believe it or not is a Magnet Mount, why because the magnet couples with the roof and creates the best ground coupling you can get.
I like the Headache-Rack with a mounting location towards the cab of the vehicle; it too has to be grounded to the frame with a decent size braided strap.
By mounting as centered as possible on the vehicle will result in a good transmission pattern, if you mount towards the rear of the vehicle, the transmission pattern will be more toward the front and in turn if mounted toward the front the pattern will go to the rear of the vehicle; toward the grounding mass.
Flippert, Nice Rack… LOL
One antenna mounted up and centered will perform better than any two antenna setup there is.
As far as tuning the radios… before you do it decide what you expect out of the CB, if only trail runs or maybe occasional road work you should not have to have anything done to the radio; the most important part is the antenna system, buy the best antenna and coax you can afford.
and keep this in mind, we are talking Theory, what works on one vehicle may not on another.
Talk to the World.
4x4, 5sp, Std Cab, Stepside, E4 Red
4” Lift - Coil-Overs, 6” SkyJacker Leafs - No Blocks (Mistake)
31 Spline FX4 Rear Axle, 4.56s and OX Locker, Hurst Shifter w/Optional Stick and "Made by Grev" alignment adapter